Genetics of Original Sin (10 page)

Here probably lies the main advantage of cell fusion. It offers opportunities for testing new combinations of genes, which may be a vital asset when genetic innovation becomes a crucial condition of survival. This is all the more true because it is during meiosis that the process called
crossing-over,
or recombination, takes place. In this process, pieces are exchanged between chromosomes of the same pair, thereby creating unique combinations of genetic material that were not present before and offering evolution an almost infinite variety of genetic motifs to play with.

Sexual reproduction represents the veritable
laboratory of evolution.
Thanks to it, innumerable genetic variants have been continually subjected to screening by natural selection (see
chapter 7
). Genesis of this mechanism no doubt constitutes a key step in the development of multicellular organisms. The complexity of this step perhaps explains why multicellular life, as we know it, was so late in appearing.

A feature of sexual reproduction common to the vast majority of plants and animals is the participation of two distinct types of germ cells with very different properties and functional roles. The female germ cells, or
oocytes,
sometimes also called (unfertilized) egg cells, are large and immobile, fitted with a full complement of cytoplasmic structures and crammed with abundant nutrient reserves and other essential substances. The role of the female germ cell is to passively await fertilization and then provide all that will be needed to start development. In contrast, the male cells, called
spermatozoa,
or sperm cells, are small and motile, reduced to little more than a nucleus
devoid of surrounding cytoplasm and propelled by an undulating tail, or flagellum. Their function is to seek a compatible egg cell and penetrate it, or, rather, insert their nucleus into it, which is all that is needed to convert the haploid egg cell into a diploid fertilized egg. A significant consequence of this mechanism is that the mitochondria of the fertilized egg are exclusively derived from the female germ cell (see
fig. 5.5
). This property is exploited in the analytical method used to trace descent by the female line (see
chapter 9
, mitochondrial Eve).

Relative to this difference in functions, male gametes are always produced in large numbers, and female gametes in very small numbers. This division of labor is energetically economical, as a female gamete is much costlier to make than a male gamete. This leaves to the sole male gametes the task, favored by their large number, of seeking a female gamete to fertilize. Many specializations of the corresponding organisms are related to the different functions of the gametes they produce (see below:
sexual dimorphism
).

Reproductive strategies have evolved very differently in plants and animals. In the latter, the haploid stage in the alternation of generations is invariably fleeting and transient, leading almost directly from meiosis to gametes through a short succession of cell divisions (maturation) that takes place in the sex glands of the male and female organisms, whereas the rest of the bodies of each sex consists entirely of diploid cells. In plants, the pathway from meiosis to gametes goes by way of an intermediate, haploid form, called a
spore,
which undergoes a variable degree of development, sometimes very complex, before giving rise to the gametes (
fig. 5.6
). This haploid stage may
be up to dominant in certain cases, while the diplod stage plays only a brief, transient role.

Fig. 5.6.
Alternation of generations in plants.
This diagram shows how plant life alternates between haploid and diploid forms, by way of spores, on one hand, and of sexual reproduction, on the other. The relative importance of the two forms varies according to the type of organism. In animals, the haploid phase is reduced to the maturation of the gametes following meiosis.

The primitive seaweeds illustrate this situation in exemplary fashion. In some species, the haploid and diploid stages have the same importance and may even be almost identical in appearance. In others, either one or the other stage is dominant, with the other stage serving only a transitional role. Mosses, the first land-adapted plants, are largely haploid and rely on a brief diploid stage to move from one haploid generation to the other. In the more evolved ferns, the situation is
reversed; the main form is diploid, and the haploid stage is only a short interlude occurring underground.

This difference between animals and plants is linked to the very different life-styles of the two types of organisms, which impose different strategies to allow the indispensable encounter between the gametes of the two sexes. Animals take advantage of their mobility to ensure this encounter. The immobile plants, on the other hand, rely mostly on external agents, such as wind or, alternatively, insects or other animals. Hence the need of a transportable form of gametes. Such is the role of spores. This function was greatly facilitated by the acquisition of a resistant, impermeable covering for the spores, allowing them to travel over considerable distances and to remain dormant for considerable times until encountering conditions favorable to germination and subsequent fertilization.

An important development, in both plants and animals, was separation between the sexes. This occurred very early in the animal line by what is known as
sexual dimorphism,
the division of reproductive functions between two distinct types of mature individuals, males and females, entrusted with the production of spermatozoa and oocytes, respectively, and endowed with appropriate specializations.

In plants, sexual separation was achieved mainly at the spore level. Male spores continued to act in dissemination, mostly in the form of pollen grains, whereas the female spores served to create a static favorable environment in which incoming male spores of the same species would selectively germinate and produce the spermatozoa needed to fertilize the locally produced oocytes. These specializations have reached an extraordinary degree of diversity in the flowering plants. Flowers are veritable traps for catching male spores, developed around the system that produces female gametes and endowed
by evolution with myriad features—shapes, colors, scents—that cause our delight but, more relevant to the plants' reproductive success, proved effective in attracting pollinators. Remarkably, most flowers also contain the male reproductive apparatus, but in a form that hinders local fertilization, thus avoiding inbreeding and the attendant perpetuation of the same genes, which is known to be genetically unfavorable.

In all higher plants, the fertilized egg develops into an immature embryo, which soon becomes arrested in its development and enclosed within a resistant casing, together with a reserve of nutrients that are to be used, upon germination, to support the further development of the embryo up to a state where it can exploit environmental resources on its own. Called
seeds,
these structures have a simple covering in the gymnosperms (
gymnos
means naked in Greek), which comprise mostly the pine trees and other conifers. In the angiosperms (from the Greek
aggeion,
covering), or flowering plants, a group that includes the majority of extant plants, the seeds are situated inside fruits, which are formations derived from the flowers and filled, under aspects of astonishing diversity, with rich nutrient stores, which serve for the nutrition of the embryo and have become, thanks to a fruit production that exceeds by far the requirements of reproduction, an abundant and succulent source of food for the animal world (including humans).

Fungi, like plants, rely mostly on spores for their dispersal. The most spectacular manifestation of fungal reproduction is represented
by the multifarious mushrooms, the spore-disseminating structures that suddenly shoot up into the open from hidden mycelia that spread their networks below the ground's surface.

Animals, taking advantage of their motility, developed a great diversity of reproductive strategies. As long as the animals kept to their aquatic birthplace, males often did little more than discharge a swarm of spermatozoa in the vicinity of females, leaving it mostly to the swimming ability of the cells and to their large number to ensure successful encounter with a female's oocytes. Most of the time, females lay their unfertilized oocytes in the same site, so fertilization and subsequent development of the fertilized egg occur in water. Cases are known, however, in which the oocytes are not discharged, but are fertilized and develop inside the female body. Some fish, called viviparous for this reason, produce progeny in this way.

With the passage to land, new mechanisms were required to compensate for the lack of water. The main such mechanism for animals was
copulation,
which is carried out by most land animals, invertebrates as well as vertebrates. The consequence of copulation is that fertilization takes place inside the female body. At first, the ancestral mode of aqueous development prevailed. Impregnated females laid their fertilized eggs in water, where embryological development occurred. Among insects, for example, mosquitoes behave in this way, which explains their predilection for swampy environments. Other insects, as well as many other terrestrial invertebrates, have evolved a great variety of reproductive strategies. Their description is beyond the scope of this book. I shall
restrict myself to the land vertebrates, which are of more direct interest to us, as the human species is one of them.

Amphibians, which were the first vertebrates to leave water, acted like the mosquitoes, quickly returning to their original medium for embryological development. Frogs offer a familiar example of this behavior. Females, after copulating on land, lay their eggs in water, where the eggs develop into swimming tadpoles adapted to aquatic life. Then, at some stage, signaled by the secretion of thyroid hormone, the tadpoles shed their tail, sprout two pairs of legs, lose their ability to derive oxygen from water, and start breathing air. The mature animals pursue their existence on land, but in the vicinity of water, where, eventually, females return to lay their fertilized eggs. This group of animals continues to thrive in all marshy lands.

The link with water was broken—or rather displaced from external to internal—by the reptiles, thanks to acquisition of a new structure of crucial importance, the
amniotic pouch,
a closed, fluid-filled sac within which fertilized eggs henceforth underwent development; they no longer needed to be laid in a body of water. Usually encased within a hard shell, the eggs could be left on land to continue their development and hatch in the open.

The reptiles bequeathed this reproductive mode to the birds and to the first mammals, the monotremes (such as the platypus), which still lay eggs. A branch then arose, in which egg-laying was replaced by birth at a very early stage of development, which was allowed to continue further within a ventral pouch, or
marsupium,
from which the young could reach
the mammary glands to feed. Thus were born the marsupials, such as kangaroos and koalas.

The last major acquisition in this saga was the
placenta,
a remarkable structure that brings in intimate proximity, separated only by the thickness of blood-vessel walls, maternal blood brought in by the mother's circulation and fetal blood conveyed via the umbilical cord, so that nourishment can pass from mother to fetus, and waste products can be unloaded from fetus to mother. Thanks to this development, which is characteristic of most of today's mammals, including humans, development was allowed to continue inside the womb up to a sufficiently advanced stage for the young to be able to pursue an independent existence (with appropriate fostering). Note that, even in this most perfected mode of development, the fetus continues development in its ancestral, aquatic mode within the amniotic pouch. Human birth, as every mother knows, is heralded by the “breaking of the waters.”

H
ow, in a matter of nine months, does a fertilized egg become the miracle that is a newborn baby? This question has been asked by generations of biologists ever since William Harvey (1578–1657), the English physician who discovered blood circulation, exclaimed, after dissecting a pregnant doe felled in hunting by his patron, King Charles I: “Omnia ex ovo,” all (living beings) arise from an egg!

The embryologists who tackled this problem found that the fertilized egg first divides into a small number of almost identical cells, which form a cluster called the
morula,
the diminutive of
morum,
the Latin word for mulberry. These are the widely publicized
stem cells,
called “totipotential” because they can give rise to any cell in the body.